Refine no-std sample.

no_std/no_std_test/Cargo.toml

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+[package]
+name = "no_std_test"
+version = "0.1.0"
+edition = "2018"
+authors = ["Stephen Chung"]
+description = "no-std test application"
+homepage = "https://github.com/jonathandturner/rhai/tree/master/no_std/no_std_test"
+repository = "https://github.com/jonathandturner/rhai"
+
+[dependencies]
+rhai = { path = "../../", features = [ "no_std" ], default_features = false }
+wee_alloc = { version = "0.4.5", default_features = false }
+
+[profile.dev]
+panic = "abort"
+
+[profile.release]
+opt-level = "z"             # optimize for size
+debug = false
+rpath = false
+lto = "fat"
+debug-assertions = false
+codegen-units = 1
+panic = "abort"

no_std/no_std_test/README.md

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+`no-std` Test Sample
+====================
+
+This sample application is a bare-bones `no-std` build for testing.
+
+[`wee_alloc`](https://crates.io/crates/wee_alloc) is used as the allocator.
+
+
+To Compile
+----------
+
+The nightly compiler is required:
+
+```bash
+cargo +nightly build --release
+```
+
+The release build is optimized for size.  It can be changed to optimize on speed instead.

no_std/no_std_test/src/main.rs

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+//! This is a `no-std` application for the that evaluates
+//! a simple expression and uses the result as the return value.
+
+#![no_std]
+#![feature(alloc_error_handler, start, core_intrinsics, lang_items)]
+
+extern crate alloc;
+extern crate wee_alloc;
+
+#[global_allocator]
+static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;
+
+use rhai::{Engine, INT};
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -> isize {
+    let engine = Engine::new();
+
+    // Evaluate a simple expression: 40 + 2
+    engine.eval::<INT>("40 + 2").unwrap() as isize
+}
+
+#[alloc_error_handler]
+fn foo(_: core::alloc::Layout) -> ! {
+    core::intrinsics::abort();
+}
+
+#[panic_handler]
+#[lang = "panic_impl"]
+extern "C" fn rust_begin_panic(_: &core::panic::PanicInfo) -> ! {
+    core::intrinsics::abort();
+}
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[no_mangle]
+extern "C" fn rust_eh_register_frames() {}
+
+#[no_mangle]
+extern "C" fn rust_eh_unregister_frames() {}